Chronic lymphocytic leukemia (CLL), a cancer of the white blood cells and bone marrow, is characterized by uncontrolled proliferation and/or reduced cell death (apoptosis) of blood cells, specifically the B lymphocytes, and is the most widespread form of adult leukemia. Many cases of CLL are detected by routine blood tests in persons with no symptoms, however, patients may have enlarged lymph nodes, enlarged liver and spleen, fatigue, bone pain, excessive sweating, loss of appetite, weight loss, flank pain, and generalized itching. Abnormal bruising, which is a more well known symptom of CLL, often does not appear until late in the illness. CLL affects the B lymphocytes (antibody producing cells) and causes suppression of the immune system, failure of the bone marrow, and infiltration of malignant cells into organs. Although CLL starts in the bone marrow, it can spread to the blood, lymph nodes, spleen, liver, central nervous system (CNS), and other organs. It does not usually form a solid mass or tumor.
The hallmark of CLL is sustained, absolute lymphocytosis (>10,000/μL) and increased lymphocytes (>30%) in the bone marrow. At diagnosis, uncommonly, there may be moderate anemia and thrombocytopenia because of bone marrow infiltration, splenomegaly, or immunohemolytic anemia and thrombocytopenia. Some patients will have hypogammaglobulinemia, and occasionally a monoclonal serum immunoglobulin spike of the same type may be found on the leukemic cell surface.
In an asymptomatic patient, CLL may be diagnosed from abnormal blood counts. Otherwise, CLL should be suspected in a patient with insidious onset of the nonspecific features mentioned above who has generalized lymphadenopathy. CLL is diagnosed by an increase in lymphocytosis and/or bone marrow infiltration coupled with the characteristic features of morphology and immunophenotype, which confirm the characteristic clonal population. Reactive lymphocytosis associated with viral infections can be differentiated by the clinical picture and the presence of atypical lymphocytes on blood smear. Confusion with other diseases may be avoided by determination of cell surface markers. For instance, CLL lymphocytes coexpress the B-cell antigens CD19 and CD20 along with the T-cell antigen. CLL B cells express relatively low levels of surface-membrane immunoglobulin (compared with normal peripheral blood B cells) and a single light chain (kappa or lambda). The cells in B-cell CLL also co-express B-cell markers, such as CD5, CD9, and CD23 (1).
Clinical staging is primarily based on hematologic changes or extent of disease, and is useful for prognosis and treatment. Accepted treatment regimens for patients with CLL consist of administration of a variety of therapeutic anti-CLL agents, including nucleoside analogs or alkylating agents, and current trials are investigating the benefits of combinations of these agents with monoclonal antibodies. However, therapeutic options for patients with CLL are limited, and in most cases, are ineffective or have a limited period of effectiveness. Relapse of the disease often occurs and these patients acquire resistance, not only to the drug used for patient treatment, but to other drugs as well.
The therapeutic approaches for CLL aim to induce or increase apoptosis of malignant B-cells. The second messenger cAMP can promote apoptosis via activation of protein kinase A (PKA) in malignant B- and T-cells; pharmacological agents that increase cAMP levels thus have the potential to treat CLL (2-4). The intracellular concentration of cAMP and PKA activity are lower in lymphocytes of CLL patients than in those of normal subjects, suggesting a disease-related defect in this pathway (5, 6).
The cellular level of cAMP is governed by the balance between its formation by adenylyl cyclases (ACs) and degradation by cyclic nucleotide phosphodiesterases (PDEs). Eleven families of PDEs that hydrolyze cAMP and cGMP have been characterized and are comprised of a number of isoforms and splice variants (7). For instance, PDE4, 7, and 8 isoforms are specific for cAMP; PDE5, 6, and 9 isoforms are specific for cGMP; and PDE1, 2, 3, 10, and 11 isoforms have dual specificity. Members of the same PDE family show 65% or greater amino acid sequence identity, whereas between families the amino acid identity drops to 40% or lower (23, 49-50). Different PDEs can be distinguished from each other by their structure, tissue expression, localization, substrate specificity, regulation, and sensitivity to PDE inhibitors (23, 50).
PDEs have unique regulatory characteristics, cellular distribution, subcellular localization and sensitivities to inhibitors that make them attractive drug targets for specific diseases. Previous studies have shown that the nonselective PDE inhibitor theophylline increases intracellular cAMP and induces apoptosis in peripheral blood mononuclear cells (PBMC) of CLL patients without affecting normal B-cells and down-regulates expression of the anti-apoptotic protein Bcl-2 in CLL-PBMC (8, 9). Addition of theophylline to chlorambucil (an alkylating agent commonly used to treat CLL) can increase the response rate and progression-free survival of CLL patients (10, 11). Because theophylline inhibits many PDEs and has a narrow therapeutic index, treatment of CLL might be aided by the identification of a PDE isoform that could be targeted for its pro-apoptotic effect in this disease.
PBMC isolated from CLL patients expresses numerous PDEs, including PDE4 isoforms that are constitutively expressed. PDE4-specific inhibitors can induce apoptosis of CLL-PBMC and to augment killing of such cells by glucocorticoids, a clinical therapy in CLL (3, 4, 13). Akin to theophylline, PDE4 inhibitors can decrease the expression of anti-apoptotic proteins (e.g., Bcl-2) and enhance expression of the pro-apoptotic protein Bax in CLL-PBMC (14). Such results suggest that PDE4 inhibitors are a potential therapy for CLL but other PDE isoforms may be a more unique targets for this disease.
PDE7 is also found to play a role in the activation and/or proliferation of T cells (27), and isoform PDE7A has been cloned and detected in lymphoid cells (B and T lymphocytes) from patients with CLL (22-23). PDE7A further has two isoforms generated by alternate splicing: PDE7A1 restricted mainly to T cells and the brain, and PDE7A2 for which mRNA is expressed in a number of cell types including muscle cells. The isoforms have different sequences at the amino termini, and it is thought that this portion of each molecule is likely to be important for cellular localization of the enzyme. However, the catalytic domain of each PDE7A enzyme is identical (51). Isoform PDE7B has also been cloned (23, 28, 52) showing approximately 70% homology to PDE7A in the catalytic domain (52). Expression of PDE7B was detected in multiple tissues, including brain, heart, pancreas, liver, and skeletal muscle, but not in lymphoid cells.
CLL is a heterogeneous disorder whose course in individual patients is quite variable (33, 34). Methods used to assess prognosis in CLL do not consistently identify stable vs. progressive forms of the disease therefore new prognostic markers could prove useful (35, 36). WBC, lymphocyte doubling time (<12 months indicating poor prognosis), and factors that include serum constituents (e.g., thymidine kinase, LDH, soluble CD23, β2-microglobulin), and features of CLL cells (e.g., p53-mutations, CD38 expression, somatic mutations in IgVH, and ZAP-70 expression) have been studied as predictors of outcome in CLL-patients (34, 37, 38-44). Unfortunately methods for measuring all such factors are not standardized and are not available in every clinical laboratory (45).
Therefore, there is a need to discover new diagnostic methods by identifying new biomarker and/or prognostic factors associated with CLL, and therapies for CLL in order to make significant progress in improving overall patient condition and survival in this disease. Given the fact that previous studies have not comprehensively compared the expression of PDE isoforms in CLL patients, it would be important and valuable to examine the expression or enzyme activities of PDE isoforms, in particular isoforms that hydrolyze cAMP, in CLL-PBMC, and further, to investigate if their inhibition would promote apoptosis in these cells. Thus, selective PDE isoforms may serve as novel biomarkers for CLL and even provide new therapeutic targets, inhibition of which would have greater efficacy to specifically kill malignant B-cells and further treating and/or curing CLL.